In recent years, with a remarkable spread of mobile phones, cordless phones, patch antennas for RFID, lens antennas such as a radio telescope, a millimeter-wave radar, and the like and significant progress of satellite communication apparatuses, a communication signal is demanded to have a high frequency, and a communication apparatus is demanded to be more compact. As the dielectric constant of a material for an antenna incorporated inside the communication apparatus becomes higher, the communication signal is allowed to have a higher frequency, and the communication apparatus is allowed to be more compact. The dielectric constant and the dielectric dissipation factor are parameters indicating the extent of a polarization inside a dielectric and the loss of an energy generated by a polarization inside the dielectric or by application of conductivity thereto respectively. Therefore the use of the material for the antenna having a high dielectric constant allows the communication signal to have a higher frequency, circuits to be shorter, and the communication apparatus to be more compact. In addition, as the use form of the communication apparatus diversifies, the material for the antenna is demanded to have a low degree of a change in its electrical characteristic at low to high temperatures, to have a low extent of dependence on a frequency in the electrical characteristic thereof, and be excellent in its flame retardance. When the material for the antenna has a low dielectric dissipation factor, power consumption at a communication time can be suppressed.
As a material for obtaining an antenna showing a high dielectric constant and a having a low dielectric dissipation factor in a wide temperature range from a low temperature to a high temperature, a dielectric elastomer composition is known. The dielectric elastomer composition contains the elastomer such as ethylene propylene rubber mixed with the barium.neodymium ceramic powder having the temperature coefficient α (unit: 1/° C.) of the dielectric constant in the range of (−200 to 100)×10−6 in the temperature of −40° C. to 100° C. (see patent document 1).
A highly dielectric elastomer composition in which dielectric ceramics and carbon black are essentially added to an elastomer whose dielectric dissipation factor is not more than 0.007 is known. In the highly dielectric ceramics, 600 to 1400 parts by weight of the dielectric ceramics and 5 to 40 parts by weight of the carbon black are added to 100 parts by weight of the elastomer (see patent document 2).
But in the patent document 2, the kind (particle diameter and the like) of the carbon black to be added to the elastomer is not specified. Further the details of the carbon black to be added to the elastomer to obtain a material showing a high dielectric constant and a low dielectric dissipation factor is described insufficiently.
As a measure for improving the flame retardance of the material for the antenna, it is known that the material for the antenna contains a halogen flame retardant such as a brominated flame retardant or a chlorinated flame retardant, for example, polybrominated diphenyl ether (hereinafter referred to as PBDE), polybrominated biphenyl (hereinafter referred to as PBB). To improve the flame retardance, antimony trioxide is usually used in combination with the brominated flame retardant other than the PBB and the PBDE.
It is known that to improve the flame retardance of the elastomer material, in addition to the halogen flame retardant, the elastomer material contains a metal hydroxide, expanded graphite. It is known that the metal hydroxide is contained in the elastomer material composing a transfer belt and the like of an electrophotographic apparatus (see patent document 3).
The dielectric elastomer composition disclosed in the patent document 1 has a low degree of a change in its electrical characteristic from low to high temperatures and an excellent dielectric property, but does not contain the flame retardant. Therefore the dielectric elastomer composition disclosed in the patent document 1 cannot be used for applications demanded to have the flame retardance.
In using the halogen flame retardant as the flame retardant to improve the flame retardance of the dielectric elastomer composition, there is a fear that at a disposal time, dioxin is generated from the halogen flame retardant. Thus the use of the halogen flame retardant is unpreferable. The PBB and the PBDE cannot be used for electric and electronic products in accordance with the RoHS directive (directive of restricting the use of certain hazardous materials in electric and electronic apparatus) issued by the European Union (EU) and enacted in January of 2003.
Although the use of the brominated flame retardant other than the PBB and the PBDE is not prohibited, as described above, the antimony trioxide is used in combination therewith. The use of the antimony trioxide is unpreferable because the antimony trioxide contains a slight amount of lead, mercury, hexavalent chrome, and cadmium as impurities. When the material for the antenna contains the expanded graphite, the flame retardance of the material is improved, but the dielectric property thereof deteriorates extremely, which is unpreferable.
When the metal hydroxide is used, as described in the patent document 3, it is difficult to obtain the flame-retardant effect unless the elastomer material contains a large amount of the metal hydroxide. When the elastomer material contains a large amount of the metal hydroxide, normally the dielectric dissipation factor becomes high. Thus it is not known to use the metal hydroxide for the material of the antenna or the like in which a low dielectric dissipation factor is demanded. When the metal hydroxide contains a large amount of impurities such as ferric oxide, calcium oxide, silicon dioxide, and the like, the dielectric dissipation factor becomes high, which is unpreferable.
As a result of the present inventors' various investigations, they have found that when the mixing amount of the metal hydroxide is large, a foaming phenomenon occurs in a molding in dependence on a surface treatment state of the metal hydroxide. When the molding foams, there occur problems that the dielectric constant fluctuates and the adhesiveness of the molding to an electrode deteriorates.
Patent document 1: Japanese Patent Application Laid-Open No. 2006-1989
Patent document 2: Japanese Patent Application Laid-Open No. 2006-290939
Patent document 3: Japanese Patent Application Laid-Open No. 2005-97493